Triton May Be Coldest Spot in Solar System

By JOHN NOBLE WILFORD, Special to The New York Times

Published: August 29, 1989

PASADENA, Calif., Aug. 28—
Surprising Triton, the largest moon of Neptune, appears to be the coldest place in the solar system, Voyager 2 scientists reported today. It also displays a glowing aurora caused by bombarding radiation trapped by Neptunian magnetic fields.

Analysis of atmospheric measurements made by the spacecraft last week determined that the temperature at Triton's mottled pink surface, presumably shaped by ice volcanoes, is about 400 degrees below zero Fahrenheit. The nitrogen and methane atmosphere is extremely thin, about one one-hundred-thousandth of the air pressure at sea level on Earth.

In reporting the temperatures at a news conference here, Dr. Roger Yelle of the University of Arizona, a member of the Voyager science team, said, ''Simply put, Triton appears to be the coldest object in the solar system we have visited yet.''

Triton and Neptune are now almost 2.8 billion miles from the warmth of the Sun. Pluto is sometimes farther from the Sun, but its elliptical orbit has now carried it closer than Neptune. Ground-based telescopic observations of Pluto and its moon, Charon, indicate that they are slightly warmer. 280 Degrees Below Zero Triton's upper atmosphere, at altitudes of 400 miles, was found to be warmer: 280 degrees below zero. This reversal of the usual pattern, Dr. Yelle said, indicated the effects not only of solar energy but also of interactions between the atmosphere and charged particles in the radiation belts that surround Neptune and encompass Triton.

Dr. Andy Chen of Johns Hopkins University's Applied Physics Laboratory said Voyager's observations of low-energy particles indicated that the envelope of magnetic energy surrounding Neptune had trapped radiation in a belt similar to the Van Allen radiation belts around Earth. The radiation contributed to the erosion of material in Neptune's rings and was chemically modifying the methane ices on the planet's moons.

Chemical reactions caused by radiation have turned the surface on one of Neptune's newly discovered satellites, 1989 N1, as black as chimney soot, Dr. Chen said. The reddish and pink hues of Triton are also the result of such chemical transformations.

Dr. Chen also said that electrons from the radiation belts, interacting with the thin outer atmosphere, were responsible for generating the aurora observed on Triton. The atmospheric glow, comparable to the northern or southern lights at the Earth's polar regions, can be seen only in ultraviolet light and is not visible to Voyager's cameras.

Of the dozens of other moons in the solar system, only Titan, the huge satellite of Saturn, has auroras. Earth's Moon is well outside the Van Allen belts and has no atmosphere, so it could not have an aurora. Auroral Signs on Neptune

Similar auroral glows have been detected on Neptune itself, but nearer the equator than the poles. Scientists said this was apparently a consequence of the Neptunian magnetic fields being tilted 50 degrees off the planet's rotation axis.

Temperatures in Neptune's upper atmosphere were measured at 261 degrees below zero. In addition to hydrogen and helium, the main constituents, Neptune's atmosphere is composed of methane and hydrocarbons, like ethane and acetylene.

Dr. James Pollack, an atmospheric physicist at NASA's Ames Research Center, described the dynamics of Neptune's upper atmosphere as inferred from the most recent Voyager data.

Ultraviolet rays from the Sun break apart and rearrange the molecules of methane that give Neptune its pale blue color. The reactions convert methane to the more complex hydrocarbon molecules, ethane and acetylene.

These hydrocarbons, Dr. Pollack said, probably drift down in the atmosphere to colder layers, where they evaporate and condense into ices. Falling deeper into warmer layers, they eventually are converted back to methane. Meanwhile, buoyant methane clouds carry more methane vapors to the upper atmosphere. Hydrogen Sulfide in Clouds

The denser cloud deck below the methane, Dr. Pollack said, is possibly composed of hydrogen sulfide ice. Some of the bright clouds seen in the planet's southern hemisphere could be plumes rising from the hydrogen sulfide cloud deck.

Scientists had dubbed one of these clouds ''scooter'' because it appeared to zip around the planet so rapidly. In fact, Dr. Pollack said, the cloud is moving very slowly in relation to the opposite motions of surrounding clouds. The scooter is actually traveling at about the same velocity as the rotation rate of the planet.

Dr. Carolyn Porco, an astronomer at the University of Arizona, said that Voyager pictures had failed so far to clarify scientific thinking on the nature of planetary rings. None of the eight moons of Neptune, six of which were sighted for the first time by Voyager, is in the same orbit with the ring material circling the planet. Thus, they could not be the ''shepherding'' satellites that theoreticians have said create the gravitational forces that prevent ring debris from diffusing.

''At the present time,'' Dr. Porco said, ''we are fishing around to see if any other ideas might work to explain the rings.'' At Least Three Rings Seen

In the most recent analysis of Voyager pictures, Neptune definitely has three rings. The outermost ring contains distinct clumps of debris reflecting sunlight, with each of the three clearly defined clumps extending about 10 degrees of a circle.

Two other inner rings have been detected, and the innermost one, which was not discovered until after Voyager passed Neptune, appears to have some dust particles extending all the way in to the cloudtops of the planet, Dr. Porco said.

Another sheet of orbiting debris was detected inside the outermost ring. Astronomers are not sure whether to call this a single ring or several rings. For now, it is being called the ''plateau,'' describing the elevated signals of light detected in that region and shown on graphs prepared by the scientists.

Voyager 2, which flew by Neptune and Triton Thursday night and Friday morning, is now more than 3.5 million miles beyond and headed toward the edge of the solar system. In parting today, Voyager's camera snapped several pictures showing both Neptune and Triton in a single frame, the rim of the giant planet in the foreground and Triton showing up as a thin crescent in the dark distance.